1. Field of the Invention
This invention relates to hot melt material delivery systems and more particularly to a switching circuit for effecting automatic switch over between two such systems without loss of continuous flow and/or continuous pressure.
2. Description of the Prior Art
High volume users of hot melt adhesive and like hot melt material typically purchase the material in 55 gallon drums and unload the drums with equipment such as described in U.S. Pat. Nos. 2,522,652; 3,031,106; 3,282,469; 4,073,409; 4,144,986 and 4,195,755. The cited patents typify drum unloaders that include a heated platen having a shape corresponding to the interior shape of a 55 gallon drum, driving mechanism for urging the heated platen into a drum and a motor driven pump for removing hot melt material after it is melted by the heated platen. The melted hot melt material is conveyed through heated hoses to a manifold from which the material is supplied to applicators such as guns, rollers, and the like.
In certain environments, such as on a high speed assembly line, it is desirable that a continuous supply of hot melt material be available, and in such environments it is customary to provide two or more drum unloaders and to operate them alternately so that while one is delivering hot melt material, a full drum can be installed in the other.
The timing of the switch over from one system to the other is critical, particularly in situations where continuous delivery of hot melt material is essential. If the system with a full drum is activated prematurely, a pressure surge can occur in the manifold to which the material is delivered and this can cause the applicators to apply excessive hot melt material during the surge. Contrariwise, if activation of the system with a full drum is unduly delayed, there can be a period of interruption of delivery of hot melt material to the applicators.